Add option ‘build-keep-log’ to enable/disable writing of build logs

Fixes #26.

doc/manual/conf-file.xml

@@ -276,6 +276,18 @@ build-use-chroot = /dev /proc /bin</programlisting>
   </varlistentry>
 
 
+  <varlistentry><term><literal>build-keep-log</literal></term>
+
+    <listitem><para>If set to <literal>true</literal> (the default),
+    Nix will write the build log of a derivation (i.e. the standard
+    output and error of its builder) to the directory
+    <filename>/nix/var/log/nix/drvs</filename>.  The build log can be
+    retrieved using the command <command>nix-store -l
+    <replaceable>path</replaceable></command>.</para></listitem>
+
+  </varlistentry>
+
+
   <varlistentry><term><literal>system</literal></term>
 
     <listitem><para>This option specifies the canonical Nix system

doc/manual/release-notes.xml

@@ -6,6 +6,26 @@
 
 
 
+<!--==================================================================-->
+
+<section xml:id="ssec-relnotes-1.1"><title>Release 1.1 (TBA)</title>
+
+<para>This release has the following improvements:</para>
+
+<itemizedlist>
+
+  <listitem>
+    <para>The creation of build logs in
+    <filename>/nix/var/log/nix/drvs</filename> can be disabled by
+    setting the new option <literal>build-keep-log</literal> to
+    <literal>false</literal></para>
+  </listitem>    
+
+</itemizedlist>
+
+</section>
+
+
 <!--==================================================================-->
 
 <section xml:id="ssec-relnotes-1.0"><title>Release 1.0 (May 11, 2012)</title>

src/libstore/build.cc

@@ -2032,6 +2032,8 @@ string drvsLogDir = "drvs";
 
 Path DerivationGoal::openLogFile()
 {
+    if (!queryBoolSetting("build-keep-log", true)) return "";
+    
     /* Create a log file. */
     Path dir = (format("%1%/%2%") % nixLogDir % drvsLogDir).str();
     createDirs(dir);
@@ -2071,7 +2073,8 @@ void DerivationGoal::handleChildOutput(int fd, const string & data)
     {
         if (verbosity >= buildVerbosity)
             writeToStderr((unsigned char *) data.data(), data.size());
-        writeFull(fdLogFile, (unsigned char *) data.data(), data.size());
+        if (fdLogFile != -1)
+            writeFull(fdLogFile, (unsigned char *) data.data(), data.size());
     }
 
     if (hook && fd == hook->fromHook.readSide)